Descriptions

The Tumalo quadrangle lies approximately 30 kilometers behind the Cascade volcanic arc and marks the southernmost extent of continuously exposed Deschutes Formation rocks. Deschutes Formation rocks in the
Tumalo quadrangle include late Miocene volcaniclastic sedimentary rocks, ignimbrites and lapillistones, and late Miocene to early
Pliocene basalts and basaltic andesites. Volcaniclastic sediment and pyroclastic flows originated from sources west to southwest of the Tumalo area and were the products of Early High Cascade arc volcanism. Most pyroclastic flows and falls had silicic compositions and silicic volcanic material dominated the sedimentary deposits. Intermittent and apparently widespread, high discharge events onto a back-arc alluvial plain deposited tuffaceous sediments, much of which became incipient soils during long periods of subaerial exposure. The close
of sedimentation was coincident with the initiation of local volcanism at approximately 5.4 Ma. Several small monogenetic shield volcanoes and cinder cones in the Tumalo area erupted the basalt and basaltic andesite lavas which cap the sedimentary section. The basalts are
typically porphyritic and high in A1203. Basaltic andesites are aphyric to sparsely porphyritic, are commonly high in FeO and Ti02, appear younger than the basalts, and are not related to the basalts by simple fractionation. Both lava types have relatively evolved compositions based on their Fe' values. An inlier of older Tertiary rhyodacite lavas known as Cline Buttes lies near the northern boundary of the study and is the only locally exposed pre-Deschutes Formation unit. The southern part of the Tumalo quadrangle marks the boundary between continuously exposed Deschutes Formation rocks to the north and Late High Cascade volcanic units to the south. After a hiatus of approximately 4 million years, deposition in the Tumalo area resumed at approximately 0.6 Ma when pyroclastic flows from the High Cascade Range flowed east and northeast into the Tumalo area to rest disconformably above the Deschutes Formation. A diktytaxitic basalt from Newberry volcano locally
overlies the pyroclastic units. Widespread, thick alluvium obscures some 16 km2 of bedrock in the study area and appears to be the result of deposition during high discharge events associated with late
Pleistocene to Holocene age glaciation of the High Cascade Range. Some thirty en echelon normal faults of the Tumalo fault zone trend between N15°-35°W and cut Pleistocene and older units in the Tumalo quadrangle. Deschutes Formation basalt and basaltic andesite vents are localized in the fault zone. This suggests volcanism was structurally controlled and that an tensional stress regime with a northeast-southwest least compressive stress orientation prevailed in the Tumalo area between the late Miocene to late Pleistocene.